Precision air chucks generally have a number of (typically three) top jaws arranged symmetrically around a center axis which are moved radially inwardly and outwardly by jaw actuators for gripping and releasing a workpiece. The jaw actuators typically have inclined cam surfaces which slide in axial movement along correspondingly inclined surfaces formed in master jaws fixedly connected to the respective work jaws in order to displace the work jaws radially inwardly and outwardly. The jaw actuators are driven by an air-operated piston. For precision air chucks, it is desirable that the work jaws be able to open and close on a workpiece with exacting repeatability, for example, of the order of 0.000050" (1.25 .mu.), and to hold the workpiece in an exact axial position under heavy loading.
An example of a conventional air chuck is shown in FIGS. 1 and 2, of which FIG. 1 is a side sectional view of the air chuck shown in plan view in FIG. 2. The air chuck has a chuck body 11 on which three master jaws 12 are mounted in symmetric configuration around a center axis X. The master jaws 12 are slidably retained in linear recesses formed in the chuck body 11 for radial movement inwardly toward and outwardly from the center axis X (indicated by the double-headed arrows in the figures). Each master jaw 12 is fixedly connected to a respective top jaw 21 by screws 23. The top jaws 21 at the front portion of the chuck body 11 are driven by the movement of the master jaws 12 within the chuck body to open and close on a workpiece (not shown). Loading on the top jaws 21 is distributed to the master jaws 12 and chuck body through tightly-fitting dowel-type pins 24 extending through matching bores formed in the master jaws and top jaws.
Each master jaw 12 has an associated jaw actuator 13 which drives the master jaw in radial movement by means of inclined cam surfaces 13a in sliding engagement with correspondingly inclined surfaces 12a formed in the master jaw 12. The jaw actuator is connected to a piston 15 movably arranged in an air cylinder formed between a rear portion of the chuck body 11 and a back cover 16. An air tube 22 is coupled to a manifold 17 at the rear portion of the chuck body, and pressurized air is introduced through air hole 22a to move the piston 15 toward the front of the chuck body, thereby driving the master jaws radially outwardly to open the top jaws 21, and through air hole 22b to move the piston 15 toward the rear of the chuck body, thereby driving the master jaws radially inwardly to close the top jaws 21. A retaining ring 18 couples the piston 15 to the air tube manifold 17, and a center seal 19 is provided at the front end of the air tube manifold. A front cover 20 is provided over each jaw actuator 13.
As illustrated in FIG. 3, it is found that the conventional air chuck, as shown in FIGS. 1 and 2, is subject to jaw actuation errors due to torquing of the master jaw 12 by the jaw actuator 13 arranged at one side thereof. When the jaw actuator 13 moves upward, the friction force of the actuator cam surfaces 13a against the corresponding surfaces 12a generates a torque which tends to push the master jaw 12 up on one side, causing a displacement error EU which can lead to misalignment of the master jaw. Similarly, when the jaw actuator 13 moves downward, the torque generated tends to push the one side of the master jaw 12 down, causing a displacement error ED. These displacement errors tend to degrade the precision repeatability of the opening and closing positions of the master jaws.
Another type of air chuck is shown in FIG. 4 having top jaws 41, master jaws 42, air cylinder cover 44, back cover 45, piston 46, and air tube 47. In this type of air chuck, a center plunger or wedger 43 has pairs of inclined cam surfaces 43a formed at symmetric positions thereon which coact with opposed inclined surfaces formed at the respective inward ends of the three master jaws 42. However, as shown in FIG. 5, this type of power chuck also has the problem of displacement errors EU and ED caused by torquing of the master jaws. Since the wedger 43 exerts frictional forces on the inward ends of the master jaws, the master jaws tend to be deflected upward and downward at their inward ends.
Another problem that occurs particularly with small diameter chucks, e.g., 2-inch or 3-inch diameter chucks, is distortion of the top jaws under loading forces. As illustrated in FIGS. 6, 7, 8, a small-diameter chuck includes a chuck body 60, top jaws 61, master jaws 62, and jaw actuators 63 as previously described. The small-diameter chuck has a relatively small mounting area available on the chuck face, thus the length of each master jaw 62 is relatively short, e.g., of the order of 0.75 to 1.0 inch. As a result, the connecting elements between the master jaw 62 and top jaw 61 are limited by the restricted space usually to a single screw 65 and two dowel-type pins 64a and 64b. As shown schematically in FIG. 8, the top jaws usually have a step 66 for seating the workpiece W on the chuck. The step 66 raises the level of the workpiece around the height of the head of the screw 65. Thus, the loading forces F from the workpiece W become applied at an intermediate height of the top jaw 61 against the head of the single screw 65. The high loading forces on the single screw can result in slight deflection LD of the top jaw from the master jaw, causing misalignment and loss of precision. Particularly in the case of a workpiece having a cylindrical shape, the deflection can reduce the narrow band of contact between the top jaw and the workpiece, thereby reducing the gripping force on the workpiece.
Moreover, the dowel-type pins 64a and 64b of the chucks are very difficult to assemble between the master jaw 62 and the top jaw 61 because the bores for the pins in the two jaw parts must be precisely the same dimension and matched in position. The top jaw 61 is often replaced or changed with other top jaw parts and may have bores that do not precisely match those in the master jaw, thereby making it difficult to assemble it to the top jaw.